1) Domain authority. Yahoo's fortunes have declined, but it was the first go-to place on the web, a directory of ALL the web (on e upon a time) and, later, a site for discussion groups, in-house content, and a lot more. So not only does Yahoo have a gazillion more backlinks pointing to it than younger websites, simply because it got started five to ten to fifteen years earlier than most websites, but also, people know Yahoo and go to it, link to it, use it. That means Yahoo answers has a LOT of people using it. Google sees and rewards that.
2) Bulk. There's a LOT of info on it. a LOT of content.
3) The "does it add value?" consideration when it comes to duplicate content. Even iF many answers are copied, the whole of each individual answer page is more unique content than not, which kicks the duplicate content penalty, and more importantly, the copied content generally answers the question -- which means it's useful to the user.
4) Peer review. Unlike most of the web, Yahoo answers gets multiple answers to each query, then lets people vote on the best answers. Whereas on the webpages the info came from, there's no "I call BS!" or "This is sound info" meter. Google likes to see that kind of rating / judgment system, because it gives users some idea whether info is trustworthy.
5) Q&A is EXACTLY the kind of content that best matches Google search queries. When people Google search, very often, they want an ANSWER. They don't want a webpage that they have to hunt through to find the answer. So, when Google looks at Yahoo, chances are, there is one -- or maybe dozens -- of pages that are nothing more and nothing less than the very question the Google user just typed, followed by peer-reviewed, precise answers to that question.
That's what the Google user wants: the answer. Yahoo answers, with its huge database of info, its active community, and its laser-tight focus on answering questions, is ideally suited to provide those answers. No wonder it ranks so well for question type queries on Google, and no wonder sites like HP have set up and tried to imitate Yahoo answers on a much smaller scale.